The proposed research is aimed at defining fundamental mechanisms responsible for the development and function of the CD4 T-cell subset. We propose studies to define the mechanisms of intrathymic commitment to the CD4 lineage, alternative signalling pathways coupled to TCR ligation of CD4 cells by superantigen or conventional antigen and the role of the CD4 molecule in regulating the response of these cells to TCR ligation. <SA1- Our approach to defining the mechanism of CD4 commitment uses thymocytes from mice deficient in a class II-dependent instructional signal. We propose experiments to test the hypothesis that committed CD4 transitional thymocytes are generated stochastically and require a CD4- dependent signal for further development. <SA2,3- We have obtained considerable evidence that two functionally distinct signal transduction pathways are coupled to the TCR using dual- reactive (superantigen/peptide) CD4 clones. We propose to further define the biochemical elements of the two pathways and to delineate the interaction between the TCR and the two types of ligand that may be coupled to the two signalling pathways. <SA4- We propose studies to extend our analyses of unresponsiveness in dual-reactive CD4 clones to the more physiological response of primary CD4 T-cells. One approach uses primary CD4 cells which express the 2B4 TCR transgene. These cells display the differential response phenotype after stimulation in vitro by superantigen (anergy) and peptide antigen (responsiveness) noted in earlier studies of dual-reactive CD4 clones. We will use this system to define the cellular and molecular mechanism of superantigen-induced unresponsiveness in primary CD4 cells. A second set of studies examines the mechanism of negative signalling by the CD4 molecule in primary CD4 T cells. We propose to define the contribution of the cytoplasmic (CYT) domain of the CD4 molecule to negative signalling in experiments which compare the regulatory effects of CD4- derived CYT domains with CD8-derived CYT domains. <SA5- Recognition of Staph enterotoxins (SEs) by CD4 T cells is usually associated with presentation of these ligands by class II products. We have recently identified a subgroup of SEs which represent exceptions to this rule. We propose studies to distinguish the potential antigen-presenting role of non-class II SEC/SEE receptors from the role of direct TCR ligation by these SEs.